It is known to the state of the art that depending on the arrangement of the indexable inserts the asymmetrical cutting force conditions lead to an unfavorable radial action of the force on the drill shank, which can cause the drilling to deviate. For this reason, many times the operation is performed with one or more support rods fastened to the drill shank, which support the drilling tool against the bore walling. The drawback is that the support rods are highly subjected to wear and besides can not prevent the machine support of the drill from being exposed to a heavy load. Also surface quality of the bore is very defective.
Therefore the DE 27 30 418 C2 proposes a drilling tool wherein the bisector of the engaged cutting edge of each indexable insert is arranged parallelly to the drill axis. As cutting inserts, preferably, indexable inserts, are used, wherein at each second cutting edge the neighboring cutting edges define an angle of 156.degree.. However, a force balancing of the drill shank in radial direction, which is supposed to be accomplished by this drilling tool, is only then possible when each time both engaged cutting edges are basically engaged over their full length. In other words, the working areas are not allowed to overlap. Such structure has the disadvantage that when the diameter of the bore is changed, a new set of cutting inserts has to be used.
In order to eliminate this disadvantage, the EP 0 054 913 B1 proposes that the mounting for the individual indexable inserts be arranged so that the radial cutting force components of the engaged cutting edge are balanced for each individual indexable insert, in that the bisectors of these cutting edges are inclined with respect to the parallel to the drill axis by an angle which depends on the effective length of the cutting edge engaged along a partial segment.
In order to balance the radial forces, in DE 27 51 255 C2 it has also been proposed to bend the radial plane of the inner cutting insert towards the rear, against the rotating direction of the drill, by a certain angle with respect to the peripherally arranged cutting insert. However, as a result the forces are merely parallelly oriented, but not balanced, since the angle of displacement can not have any influence on the size of the radial or cutting forces. This way the forces are maintained as a function of the cutting conditions and are independent from the geometry of the tool.
Finally in the DE 38 02 290 C1 dealing particularly with drilling through stacks of plates but also through a single piece solid metal material, it has been proposed to arrange a first indexable insert radially outside on a diameter beyond the half defined by the drill axis and on the opposite side to arrange two smaller indexable inserts, whereby the effective outer cutting edge of the indexable insert arranged radially outside runs approximately on the same working cone and the same maximal radius as the radially outer cutting edge of the largest indexable insert. This arrangement is also supposed to achieve merely a balance of the radial cutting forces.